Why Do Leaves Turn Yellow on Cottonwood Trees?

Cottonwood trees can be found next to lakes.

Cottonwood trees (Populus deltoides, Populus angustifolia, Populus trichocarpa and Populus fremontii) grow natively in the Mediterranean climate of U.S. Department of Agriculture plant hardiness zones 8 and 9. You can find them growing near water in moist, well-drained soils. They are tolerant of many soil types, drought conditions and air pollution. They grow quickly, making them useful as windbreaks and shade trees despite the fact they are not particularly sturdy or durable. In addition, University of California at Davis advises that although cottonwoods are "not considered weedy in natural systems … in urban settings, [they] may become too dense and create a fire hazard or restrict movement of animals or humans." Furthermore, cottonwood trees are subject to several conditions that cause the leaves to turn yellow and mar your landscape if not properly managed.

Seasonal Change

Triangular, shiny green leaves adorn cottonwood trees during the growing season. Like all deciduous trees, cottonwoods are subject to seasonal color change in the fall. When cooling temperatures stop the leaves' production of chlorophyll, cottonwood leaves will turn a bright yellow-gold before dropping.

Iron Chlorosis

Cottonwood is sometimes susceptible to iron deficiency, which reduces chlorophyll levels and turns the leaves prematurely yellow. Without treatment, your trees' health deteriorates and they eventually die. One cause is a soil pH above 7.0 that interferes with the trees' ability to absorb iron from the soil, according to the University of Missouri Extension. A soil test will tell you if the soil pH could be the problem, although it is difficult to adjust soil pH to the depths necessary for spreading tree roots. Spraying with an iron sulfate foliar spray when the tree is in full leaf provides a temporary solution. Adding iron-rich soil enhancements around the base of the tree provides some protection against iron chlorosis for up to three years. Larger trees respond well to direct injections of iron into the tree trunk by a professional arborist. This treatment can last up to five years.

Leaf Diseases

Leaf rusts and spot diseases commonly infect cottonwood trees with fungi that leave yellowish markings on the leaves. Infected leaves can drop prematurely but may not cause long-term damage to the trees. Overhead watering contributes to fungal spore germination, so water around the base of the tree early in he day to allow the foliage to dry out. Prune infected shoots and branches as soon as they appear. Gather and remove infected leaves and debris from around the base of the tree. According to UC Davis' Agriculture and Natural Resources, fungicides are not a very effective or necessary measure against the cottonwoods' leaf diseases.

Leaf Scorch

Cottonwoods are also vulnerable to leaf scorch, which turns the leaves yellow and brown along the margins and tips due to poor irrigation and drainage, over-fertilization, drought, excessive heat or light, mineral deficiencies, salt toxicity and windy conditions. These factors can contribute to an insufficient root system that prevents the tree from taking up enough moisture and thus dehydrating the tree and turning the leaf yellow. To prevent leaf scorch, deeply water the entire root area of your cottonwoods once a month during winter to prevent dehydration. During spring and summer, water deeply and infrequently, allowing the soil to dry slightly between waterings to encourage oxygen circulation. You cannot cure leaves that have already been scorched, but with proper watering practices, any new growth should come in green and healthy.

Age

Ron Smith, a horticulturalist with the North Dakota State University Extension Service advises that cottonwoods are known for developing problems as they age. He recommends you have a certified arborist inspect older trees at the earliest sign of yellowing symptoms so the problem can be corrected easily and economically.

About the Author

Tamara Christine has written more than 900 articles for a variety of clients since 2010. She holds a Bachelor of Arts in applied linguistics and an elementary teaching license. Additionally, she completed a course in digital journalism in 2014. She has more than 10 years experience teaching and gardening.